Bik, also known as nbk, is one of the pro-apoptotic BH3-only proteins, which have only one of the Bcl-2 homology regions, BH3 domains, and have recently been recognized as essential initiators of apoptosis (Han et al., 1996; Boyd et al., 1995). Loss of informative alleles on chromosome 22q where the Bik gene is located may be related to the development of human breast and colorectal cancers (Daniel et al., 1999). The 18-kDa Bik protein interacts with E1B 19K and forms heterodimers with various anti-apoptotic proteins, e.g. Bcl-2 and Bcl-XL, the association of which hinders the function of the anti-apoptotic protein (Han et al., 1996). Bik-mediated apoptosis requires BAX (Theodorakis et al., 2002) and is independent of p53 (Han et al., 1996). Bik is also a downstream apoptotic effector of p53 (Bartke et al., 2001) in response to a physiological p53-mediated death stimulus provided by E1A. Elevated Bcl-2 functioned downstream of p53 and Bik induction to inhibit the E1A death pathway, with the ratio of anti-apoptotic Bcl-2 and pro-apoptotic Bik determining cell death or survival in E1A-expressing cells (Mathai et al., 2002). Moreover, Bik can sensitize tumor cells to certain chemotherapeutic agents (Daniel et al., 1999) and one of the chemotherapy drugs, doxorubicin, that induces apoptosis is mediated by Bik gene (Panaretakis et al., 2002). All of these suggest that Bik is a useful therapeutic gene to target human cancer.
BH3-only proteins differ in their expression pattern and mode of activation, and many BH3-only proteins activation needs posttranslational modification (Puthalakath and Strasser, 2002). Bik is one of the BH3-only proteins whose activity can be regulated by phosphorylation (Verma et al., 2000). Verma et al. demonstrated that Bik exists as a phosphoprotein and is phosphorylated at residues threonine 33 and serine 35, which they determined is required for the full apoptotic activity of Bik, possibly by a casein kinase II-related enzyme. That is, Verma et al. mutated the phosphorylation sites at the threonine and serine residues to alanine residues, which reduced the apoptotic activity of Bik. They concluded phosphorylation is required for the pro-apoptotic potency of Bik by a presently unknown mechanism without significantly affecting its affinity for Bcl-2.